My heater won't work on my 71 mav, A/C works great and all the other functions are fine on the control panel. I checked my heater control valve and noticed it was kinda stuck so I turned on the heater and manually pushed down the lever on the control valve and the heater worked great. Do I need to change the control valve? Is there a cheaper one than this $70 piece? Also, is it hard to change? Because it looks like I'd have to drain the coolant.